Regulated power supplies are used in electronic devices and apparatuses. The core of so called switching power supplies that offer a more efficient conversion than classical linear transformer-based power supplies is a switched DC-DC converter. Switched DC-DC converters may be realized with one of many circuital topologies, the choice of which may be dictated by design considerations, convenience or specific requirements.
Generally, hard switching converter topologies give rise to transient signals, the spectrum of which contains high frequency components that may have effects of electromagnetic emissions, introducing noise in the area surrounding the converter circuit. The noise may interfere with information carrying signals or the like imposing the use of additional circuits for preventing spurious interferences with the useful signals or to be compliant with electromagnetic emission standards and rules.
Energy conversion efficiency has paramount importance in most applications and the choice of the circuit topology of the switching converter and its mode of operation, most often implemented by a dedicated control circuit of the switchings, may be carefully chosen in a way to satisfy the application requirements of low noise generation with attendant maximization of efficiency. To this aim multimode switching control converters have been devised, in many of which the dedicated logic automatically switch from one mode to another mode of operation depending on current conditions of operation in terms of input DC voltage of the converter and/or of load conditions (output current delivered by the converter).
A dual-mode power factor correcting converter of this kind is disclosed in U.S. Pat. No. 6,172,492, wherein the converter operates at variable switching frequency, in practice in a quasi resonant (QR) mode when delivering power, (primary mode of operation) and in a fixed off-time (FOT) mode when the converter is in stand-by or in a case delivering a comparably small power (stand-by mode).
In a so-called “flyback” converter that typically employs a transformer, under full load conditions and in presence of a relatively low input DC voltage, a large part of the losses are imputable to conduction losses of the rms current through the primary winding of the transformer and through the power switch connected with it that when the flyback converter operates in a quasi resonant (QR) mode increase sensibly.
In a flyback converter operating in QR mode, the switching frequency decreases when the input DC voltage, Vin, diminishes. It is easily demonstrated that for the same output power, if the switching frequency of the converter decreases, the peak current at the primary (IPK) may increase such to verify the equation
      P    O    =            1      η        ·    LP    ·          I      PK      2        ·          f      SW      
If IPK increases, the rms current flowing in the primary winding of the transformer and therefore in the power switch increases, thus increasing the conduction losses according to the following equations (D=duty cycle):
            I      RMS        =                  I        PK            ·                        D          3                                P              Loss        ⁢        _        ⁢        Cond              =                  R                  DS          ⁢          _          ⁢          ON                    ·              I        rms        2            
On the other hand, operation in a quasi resonant (QR) mode has the advantage of reducing switching losses in the power device (often a power MOSFET) that can be significant in case of high input voltage.